Process of producing steel



Oct. 29, 1957 R. P. HEUER PROCESS OF PRODUCING STEEL Filed Feb. 8, 1954INVENTOR. fimssef/ 1. nswer United States Patent PROCESS OF PRODUCINGSTEEL Russell Pearce Heuer, Villanova, Pa.

Application February 8, 1954, Serial No. 408,806

Claims. (Cl. 75-52) The present invention relates to the manufacture ofsteel which has improved properties, particularly improved ductilitywhich renders the steel especially suitable for drawing and coldforming.

A purpose of the invention is to secure a steel for cold drawing andforming which has a maximum degree of uniformity and freedom fromphysical imperfections, so that drawing and forming operations ofgreater severity can be performed and operations involving larger areasin the sheet undergoing mechanical work, with fewer imperfections in theproduct.

A further purposes of the present invention is to manufacture steelsuitable for improved drawing and cold forming from pig iron containingless than 0.013 percent sulphur by weight, using a combustion furnace inthe absence of external sulphur in the furnace to obtain a steel havingan unusually low sulphur content, thereby avoiding the objectionablesegregation of sulphur compound which occurs in steel which is made inconventional combustion furnaces and also avoiding the adverse effect ofsuch segregation on the deep drawing and cold forming properties of thesteel.

A further purpose is to desulphurize molten pig iron after it has beenproduced in the blast furnace and has left the blast furnace, and toconvert the pig iron thus produced into steel under conditions freefrorncontamination with sulphur by using a converter in. which metalloids inthe steel are oxidized with relatively pure oxygen gas (containingsubstantially more oxygen than air) which is brought into contact withthe carbon, silicon. and other oxidizable constituents of the iron toyield the necessary heat.

A further purpose is to produce steel from pig iron by desulphurizingmolten pig iron external 'to the blast furnace inv a closed space underreducing conditions with exclusion of air. and in the presence of adesulphurizing agent until the sulphur. content is below 0.013 percentby weight, then to convert the desulphurized iron into steel in thepresence of a basic slag and in the absence of external sulphur-bearingfuel by blowing with oxygen containing substantially less nitrogen thanair and thus producing combustion until 85 to 95 percent or more byweight of the carbon present is removed as required, and then toseparate the slag from the steel. I

A further purpose is to convert the ironto steel using oxygen containingless than 4 percent of impurities by volume and to continue the blowinguntil the carbon content is less than 0.10 percent by weight. g

A further purpose is to desulphurize by introducing lime blown in withnitrogen gas.. 7

A further. purpose. is to blow oxygen on the surface of the iron from atuyere. under a pressure of 50 to 175 p. s. i. gage in the converter.

A further purpose is to desulphurize until the iron contains less than0.005 percent sulphur by weight.

A further purpose is to produce a steel containing less than 0.010percent sulphur by weight and preferably less than 0.004 percent sulphurby weight.

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Further purposes appear in the specification and in the claims.

The drawings illustrate mechanism which may be used in carrying out theinvention.

Figure 1 is a central vertical diagrammatic section of a converter whichmay be employed in the invention.

Figure 2 is a section on the line 22 of Figure 1.

The conventional practice in the manufacture of steel which is requiredto have highest ductility and especially of steel intended to be drawnor cold formed is to use the basic open hearth process.

It is preferred over the acid open hearth, the Bessemer and the Thomasprocesses because the product has a lower content of sulphur andnitrogen. The sulphur content is especially critical for sheet steelwhich is to be deep drawn. Even sheet from the basic open hearth processleaves much to be desired in this respect.

In basic open hearth steel sheet the sulphur compounds are notdistributed uniformly during solidification of the ingot, and thesoaking pit and rolling operations fail to overcome the difliculty dueto segregation. The nonuniform distribution of sulphur compounds in thefinal sheet causes many difficulties in forming. As an example, in theforming of automobile fenders, the presence of one objectionableimperfection on the surface of the sheet or one local failure of thesteel to withstand the forming operation. causes rejection of the entireproduct, even though the imperfection may apply to a very small part ofthe entire piece, for example, one-ten-thousandth of the entire piece.

The limitations of the steel sheet furthermore greatly restrict theseverity of the drawing operations and necessitate that more expensiveand prolonged drawing operations be used in order not to exceed thelimited ability of the steel sheet to withstand forming.

One ofthe great advantages of the present invention is that steel sheetcan be obtained which has a lower total sulphur content and lesspronounced segregation of sulphur. Thus in accordance with the inventionit is readily possible to produce steel sheet having a sulphur contentof less than 0.013 percent sulphur by weight, and in many cases thesulphur content is as low as 0.004 percent sulphur by weight or less.

In accordance with the present invention, molten pig iron after itleaves the blast furnace, normally containing about 0.025 to 0.050percent sulphur by weight, or even more, is placed in a suitabledesulphurizing vessel With exclusion of air, and in the presence of adesulphurizing agent, preferably lime. It is best to desulphurize in anunheated vessel, decreasing the activity of oxygen in the vessel byblowing a small quantity of an inert gas such as nitrogen through themolten iron to remove the carbon monoxide present as taught in Wynne U.S. Patent No. 2,587,573, granted February 26, 1952, for DesulphuriziugProcess. p

The desulphurizing agent may be for'example a molten slag whose primaryactive ingredient is lime, with suitable amounts of silica and/ oralumina and fiuorspar. desulphurizing agent may also be used, forexample lime powder in an unfused state, and preferably blown into themolten iron by a stream of nitrogen gas. The use of nitrogenindesulphurizing will have no adverse effect on the quality of the steelultimately obtained by the present process.

Powerful reducing conditions are built up during the desulphurization bythecarbon content of the pig iron when air is excluded, and in thepresence of a desulphurizing agent as already explained, the sulphur ofthe pig iron can be reduced to less than half the initial content, sothat the molten pig ironv after desulphurization will contain less than0.013 percent sulphur by weight and preferably less than 0.005 percentsulphur by weight, and

A solid' most desirably of the order of 0.004 percent sulphur by weightor less.

From the ladle or other desulphurizing vessel, the desulphurized iron istransferred to a basic lined converter. Figures 1' and 2 show a suitableconverter 20 having a casing 21 and an interior basic refractory lining22 which may consist of tar-dolomite brick or magnesite brick. Theconverter has a lower chamber 23 and is reduced in cross section at theneck 24, and open at a blowing opening 25 at the top. The converter willbe suitably adjusted for tilting as well known. A charge of molten pigiron 26 is brought into the converter from the desulphurizing step, and.the charge is blown with relatively pure oxygen.

The oxygen content of the blowing gas should be substantially greaterthan that of air, although it may be obtained by enriching air inoxygen. Advantage from the invention will be obtained by using more than50 percent of oxygen by volume in the blowing gas, the balance beinginert impurities such as nitrogen, although great advantage is obtainedby using substantially pure blowing gas, containing at least, 96 percentof oxygen by volume and preferably from 98 to 99.5 percent of oxygen byvolume, the balance being inert impurities such as nitrogen. The use ofthe relatively ure oxygen is advantageous not only because. of the highcalorific intensity of the reaction obtained, but also because of thelow nitrogen content which tends to produce a steel unusually low innitrogen.

The effect of the blowing is to remove the silicon from the pig iron andreduce the carbon content to a. suitably low figure, for example lessthan 0.10 percent by weight and usually to 0.05'percent carbon by weightor less. At least 85 percent and preferably at least95 percent by weightof the carbon present in the original pig iron is removed at this stage-The phosphorus is removed to about 0.040 percent or less.

The desirable technique is not to blow the oxygen containing gas throughthe molten metal bath from bottom tuyeres as in the conventional Thomasprocess. Best results according to the present invention are obtained byblowing the oxygen containing gas upon the surface of the iron suitablyfrom an external vertical tuyere 27 shown in Figures 1 and 2. Thevigorous gas flow is important, and it is recommended that the pressureof the oxygen containing gas be between 50 and 175 p. s. i. gage at thetime the gas. is released by the tuyere.

The slag which is on the molten iron at 2 8 is made basic by theaddition of sufiicient lime to facilitate re.- moval of phosphorus. Theratio of lime to silica in this slag 28 is from about 2 to about 3.5 to1, and the total content of lime plus silica will be 50 percent or moreby weight. The total content of FeO plus MnO in the slag will be betweenabout 25 and 40 percent by weight. a

If desired limited amounts of scrap may be added tothe converter chargeif the sulphur content of the scrap is suitably low. The preferred scrapis mill and fabricating scrap arising from the working of the steelproduced by the process of the present invention.

One great advantage of heating by the oxygen reaction. with themetalloids in the pig iron of the converter is that; by this proceduresulphur pick-up from combustion gases usually encountered in ordinarycombustion furnaces like the open hearth furnace is avoided, and alsothe excessive nitrogen content commonly present inconverter steel fromthe Thomas or Bessemer processes are avoided.

It will be evident that the low sulphur content is obtained by acombination of a metallic charge in the converter having a very lowsulphur content and avoidance of sulphur pick-up from the furnace gases.or other mate-. rials used in the converter. The slag in the converteris not. relied upon as the principal cause. of the low sulphurcontentthe finished steel. In. fact he cqnve rter slag at the end of theoperation of steel making may contain lessthan 0:1" percent sulphur byweight.

The steel produced in the present invention has a sulphur content whichis not more than 0.010 percent sulphur by weight and preferably aslittle as 0.004 percent sulphur by weight.

The nitrogen content of the finished steel will depend upon the nitrogencontent of the oxygen blown by the tuyere 27. By using oxygen whichcontains less than 3 percent nitrogen and preferably less than 1 percentnitrogen by volume, thesteel product obtained; contains about 0.005percent nitrogen by. weight or less. In some cases the nitrogen contentof the converter steel of the present invention may somewhat exceed thenitrogen content obtained in conventional open hearth steel, but thecombined effect of the unusually low sulphur content as compared Withopen hearth steel and the moderate nitrogen content makes the product oithe present invention exceptionally valuable, especially for deepdrawing and cold forming.

It will be evident that the end of the blow may be determined by anysuitable means, either by visual inspection, automatic control, oranalysis as well known. After the bl'ow'is complete, the molten steel ispoured from the converter to a ladle or the like, while the slag ispreferably retained in the converter. The composition of the steel maybe adjusted in the ladle by desired additions of ingredients well knownin the art, such as carbon, manganese, silicon, and. other alloys, withor without ingredients which control oxide content such as aluminum, asrequired in normal practice to make killed steel, semi-killed, cappingor rimming grade. The process of the. Present invention is especiallyadvantageous for making capping or rimming steels intended for deepdrawing where a minimum of croppingis practiced to remove the segregatedcap of the ingot, since in accordance with the present inventionsegregation of sulphur in the ingot is practically eliminated.

It will be evident that by using low sulphur raw materials, and refiningthe steel in a combustion furnace in the absence of sulphur, whileavoiding excessive nitrogen, I obtain a steel of maximum uniformitywhich is remarkably free from physical imperfections, and which hasunusual ductility and is, capable of most eflicient cold 7 forming withminimum waste and rejection.

In view of my invention and disclosure, variations and modifications tomeet, individual whim or particular need will doubtless become evidentto others skilled in the art, to obtain all or part of the benefits ofmy invention without copying the, process shown, and I therefore claimall such insofar as they fall within the reasonable spirit and scope ofmy claims.

Having thus described my invention, what I claim as new. and desire tosecure by Letters Patent is:

1. The process of producing. steel of improved ductility, which.comprises desulphurizing moltenv pig iron external to the. blastfurnace, in a closed space. under reducing conditions intensified by aninertnitrogen atmosphere and in the presence of a desulphurizing agentuntil the sulphur content. is below 0.0.13 percent'by weight, convertingthe desulphurized iron into steel in the presence of a basic slag and,in the absence of sulphur-bearing fuel by blowing with oxygen containingsubstantially less nitrogen than that in the air and thus oxidizingimpurities liberating voluminous carbon. monoxide and, thereby removingnitrogen, the. sulphur. content at the end of blowing being still lessthan 0.013 percent by weight, and separating the slag from the moltensteel.

2. The process of claim 1,. which comprises blowing the pig iron withnitrogen during desulphurizing.

.3. The process of claim 2, which comprises blowing in line withthenitrogcn dutingdesulphurizing.

4. The process of claim, 1, in which the oxygen con- 3 talu less han. 4-nercent. of impurities. by vol mee p ss'ofi cla m 1.; in-which he. pigiron s;

5 desulphurized until it contains less than 0.005 percent sulphur byweight, and a sulphur content below this limit is maintained at the endof blowing with oxygen.

6. The process of claim 1, in which the iron is contained in a basiclining during conversion into steel, and the oxygen is blown upon thesurface of the iron from a tuyere under a pressure of 50 to 175 p. s. i.gage.

7. The process of claim 1, in which the steel contains less than 0.010percent sulphur by weight.

8. The process of claim 7, in which the steel contains less than 0.004percent sulphur by weight.

9. The process of claim 1, in which the steel contains not more than0.005 percent nitrogen by weight.

10. The process of claim 1,' in which the charge for conversion intosteel is partly liquid metal and partly solid metal.

6 References Cited in the file of this patent UNITED STATES PATENTS1,032,653 Brassert July 16, 1912 2,290,961 Heuer July 28, 1942 2,644,746Hauttmann July 7, 1953 2,741,555 Cuscolcca et al Apr. 10, 1956 FOREIGNPATENTS 19,727 Great Britain of 1892 OTHER REFERENCES Metal Progress,vol. 64, No. 1, July 1953, pages 142, 144, and 176. (The foregoingpublication is a digest of Blowing Thomas Pig Iron With Pure Oxygen,published in Stahl and Eisen, vol. 73, Jan. 1, 1953,) Date relied on,Jan. 1, 1953.

1. THE PROCESS OF PRODUCING STEEL IMPROVED DUCTILITY, WHICH COMPRISESDESULPHURIZING MOLTEN PIG IRON EXTERNAL TO THE BLAST FURNACE IN A CLOSEDSPACE UNDER REDUCING CONDITIONS INTENSIFIED BY AN INERT NITROGENATMOSPHERE AND IN THE PRESENCE OF A DESULPHURIZING AGENT UNTIL THESULPHUR CONTENT IS BELOW 0.013 PERCENT BY WEIGHT, CONVERTING THEDESULPHURIZED IRON INTO STEEL IN THE PRESENCE OF A BASIC SLAG AND IN THEABSENCE OF SULPHUR-BEARING FUEL BY BLOWING WITH OXYGEN CONTAININGSUBSTANTIALLY LESS NITROGEN THAN THAT IN THE AIR AND THUS OXIDIZINGIMPURITIES LIBERATING VOLUMINOUS CARBON MONOXIDE AND THEREBY REMOVINGNITROGEN, THE SULPHUR CONTENT AT THE END OF BLOWING BEING STILL LESSTHAN 0.013 PERCENT BY WEIGHT, AND SEPARATING THE SLAG FROM THE MOLTENSTEEL.